The invention relates to an ultrasonic transducer of the type which has a piezo-electric transducer layer, a first adaptation layer coupled to the piezo-electric transducer layer, and a second adaptation layer which is coupled to the first adaptation layer and in operation is turned toward an object to be examined.
Ultrasonic transducers of this kind are widely used in medical technology to obtain information about the internal structures of tissues and organs in a patient. One problem area is how to introduce the ultrasonic waves into the patient.
The piezo-electric transducer used in medical ultrasonic antennas is often made of a material which has a relatively high acoustic impedance. Such materials as lead-zirconate-titanate ceramics have, for example, an acoustic impedance of about 30.times.10.sup.6 kg/m.sup.2 s. The patient's skin and tissue, however, has only an acoustic resistance of about 1.5.times.10.sup.6 kg/m.sup.2 s. To avoid an undesirably large reflection at the interface between the piezo-electric transducer layer and human tissue, an adaptation (or impedance-matching) layer is disposed between the transducer and tissue.
A single adaptation layer of a plastic with an acoustic impedance of about 3.times.10.sup.6 kg/m.sup.2 s or slightly more has been used to match the acoustic impedance of the ceramic transducer to that of the object to be examined (e.g. human tissue with an impedance of about 1.5.times.10.sup.6 kg/m.sup.2 s). This adaptation layer had a thickness of .lambda./4, .lambda. being the wavelength that exists in the material in accordance with the nominal frequency of the ultrasonic transducer. A theoretically favorable value is 7.times.10.sup.6 kg/m.sup.2 s when transforming down from 30.times.10.sup.6 kg/m.sup.2 s (ceramic) to 1.5.times.10.sup.6 kg/m.sup.2 s.
The disadvantage of using a single adaptation layer is that the bandwidth is not wide enough. To obtain high penetration depths and good axial resolution over a large frequency range, a first and a second adaptation layer of .lambda./4 thickness each have been used (cf. Biomedizinische Technik, Volume 27, No. 7-8, 1982, p. 182-185). The acoustic impedances of these two adaptation layers are about 12.times.10.sup.6 kg/m.sup.2 s for the first adaptation layer (which faces the piezo-electric ultrasonic transducer) and about 4.2.times.10.sup.6 kg/m.sup.2 s for the adaptation layer which faces the tissue or patient. Thus a much better adaptation can be obtained.
Materials for the second adaptaton layer with an acoustic impedance of about 4.2.times.10.sup.6 kg/m.sup.2 s are easy to find or to produce. Common plastics may be used. Since the impedance of the second (plastic) adaptation layer advantageously to be used is substantially independent of the impedance of the ultrasonic transducer ceramic, the impedance once selected is equally suitable for all PZT ceramics of the ultrasonic transducer.
On the other hand, it is difficult to find materials for the first adaptation layer. This should have a mean acoustic impedance that should to some degree be adjustable because of its (theoretically corroborated) dependence on the impedance of the piezoceramic piezo-electric transducer layer with which it is used. Under the conditions named it should be about 12.times.10.sup.6 kg/m.sup.2 s. With natural materials such an acoustic impedance is difficult to obtain. Gases and liquids, for instance, are in the range of 0 to 4.times.10.sup.6 kg/m.sup.2 s. Above the last-named value there is a certain gap, i.e. materials with such an impedance are practially non-existent, and the values of minerals, metals, etc. range between 14 and about 100.times.10.sup.6 kg/m.sup.2 s. Materials having acoustic impedance of around 12.times.10.sup.6 kg/m.sup.2 s can be fabricated only with great difficulty, using glass compounds. As a rule, borosilicate glass is used. The use of this and other glasses, however, entails a number of disadvantages. The fabrication of glass is time-consuming and expensive. Moreover, some glasses are toxic in the impedance range in question; they must therefore be treated before they can be used. It has now been found that the first adaptation layer has an especially great influence on the quality of the ultrasonic picture.
One object of the invention is to provide a material for a first adaptation layer which can be adjusted to a desired acoustic impedance during manufacture and which has mechanical properties that permit relatively easy fabrication.